1. Field of the Invention
This invention relates to an oxygen annealing process for bulk superconductors, and more particularly to a process capable of quickly and sufficiently introducing oxygen into a bulk oxide by taking a newly-devised treatment in the introduction of oxygen into the bulk oxide.
The bulk superconductor intended in the invention is a REBa.sub.2 Cu.sub.3 O.sub.y bulk superconductor suitable for use in superconducting bearings, superconducting transport systems, superconducting magnets, magnetic shields and the like.
2. Description of Related Art
Recently, a large-size bulk superconductor is desired as a part of applications of a superconductor such as a flywheel, power application in a linear motor car, and the like.
For such bulk superconductor applications, a bulk oxide of REBa.sub.2 Cu.sub.3 O.sub.y system (123 type) is particularly considered to be useful.
Since the repulsive force between the superconductor and permanent magnet required in the above applications is proportional to the magnitudes of critical current density (J.sub.C) and shielding current loop (R), it is required to make J.sub.C and R large in order to produce a bulk superconductor having higher performances.
It is necessary to eliminate factors resulting in weak links such as grain boundaries obstructing current flow, cracks and the like as far as possible for increasing J.sub.C.
Further, it is necessary to make a large crystal for increasing R.
As a method of producing such a bulk superconductor, melt growth processes such as MPMG processes M. Murakami, Supercond. Sci. Technol., 5(1992), 185!, OCMG process S. I. Yoo et al., Appl. Phys. Lett., 65(1994), 633! and the like are employed.
The melt growth process is a method wherein a superconducting precursor (green compact body or sintered body) is heated to a temperature above a melting point of 123 type superconducting phase and then recrystallized, whereby a large-size crystal without weak links can be obtained. However, it is required to change an oxygen partial pressure in the melt growth in accordance with the kind of rare earth element (RE) used. Because, in case of a 123 type material having a RE ionic radius larger than that of Gd, if a sample is prepared in an atmosphere having a higher oxygen partial pressure, Ba ion is substituted by RE ion to deteriorate the superconducting property.
The melt-grown superconductor is a mere semi-conductive material in the as-grown state in which the oxygen content y is about 6.1-6.4 as it is, so that the oxygen content is necessary to be raised to y=about 6.9 in order to provide a good superconducting property.
Therefore, it is intended to provide the superconducting property by subjecting the resulting bulk oxide to a heat treatment at temperatures of about 300-600.degree. C. in an oxygen atmosphere for a long time, which is referred to as oxygen annealing.
According to the calculation of trapped magnetic field, the magnetic flux density becomes large as the size of the bulk body becomes large. In fact, however, oxygen required for the provision of the superconducting property is not introduced sufficiently into the inside of the large-size bulk body, so that a good superconducting property is not obtained as might have been expected.
That is, oxygen diffuses from the surface of the bulk body into the inside thereof in the above oxygen annealing, but as the size of the bulk body becomes large, it is difficult to uniformly introduce sufficient oxygen into the inside of the bulk body, so that the good superconducting property as expected is not obtained by only increasing the size of the bulk body.
From the measurement of the trapped magnetic field, it is readily confirmed that the surface of the bulk body is superconducting, but the inside thereof is only semiconducting.